Therapy initiated to treat serious infections until test results are available; or therapy for minor infections (URI’s, UTI’s) because causative organisms are predictable and their susceptibility to available antibiotics is known

Definitive Therapy

Start broad spectrum antibiotics before the culture and sensitivity results are reported. When they become reported, you know what will kill the bug in vitro and can tailor the therapy making it less broad and more definitive.

The hospital antibiogram can assist with empiric or definitive therapies.

Antibiogram

The cumulative results of hospital performed bacterial susceptibility testing organized into a summary table which may be used by clinicians, pharmacists, infection control personnel and microbiologists as a reference guide to community or hospital-specific resistance patterns.

How to Read an Antibiogram

Coming soon….

Bacterial Taxonomy

Two part names: Genus – species

E.g. Staphylococcus aureus

Grouped according to morphology and biochemical/metabolic differences

E.g. staining, oxygen use, growth req’s, enzymes, etc.

We cannot name the organism until we

Visualize it

Describe its features

Early descriptions of a microbe’s morphologic features and metabolic characteristics are helpful

Selected Medically Important Bacteria

Gram POSITIVES

Gram (+) Facultative Anaerobic Cocci

(Catalase +) – ALL staphylococcus spp.

Staphylococcus aureus (Coagulase +)

Staphylococcus epidermidis

Staphylococcus saprophyticus

Alpha-hemolytic

Streptococcus pneumoniae

Beta-hemolytic

Streptococcus pyogenes (Group A)

Streptococcus agalactiae (Group B)

Gram (+) Anaerobic Spore-formingRod

Clostridium difficile

Gram (+) Facultative Non-spore forming Anaerobic Rod

Listeria monocytogenes (intracellular parasite)

Gram NEGATIVES

Gram (-) Anaerobic Rods

The “Non-Enterics”

Haemophilus influenzae

Legionella pneumophila

Pasteurella multocida

Pseudomonas aeruginosa (Oxidase +)

Gram (-) Facultative Anaerobic Rods (generally, the “Enterics”)

Escherichia coli

Campylobacter jejuni

Helicobacter pylori (Urease +)

Salmonella

Shigella

Klebsiella pneumoniae

Gram (-) Aerobic Diplococci

(Catalase + and Oxidase +)

Moraxella catarrhalis

Neisseria gonorrhoeae

Neisseria meningitidis

Gram (-) Coccobacillus

Haemophilus ducreyi

Gram (-) Spirochete

Treponema pallidum

The “ATYPICALS”

Mycoplasmas

Mycoplasma pneumoniae

Ureaplasma urealyticum

Chlamydiae

Chlamydia pneumoniae

Chlamydia trachomatis

OTHERS

Aerobic Acid-Fast Rod

Mycobacterium tuberculosis

Staining and Morphology

Staining

Gram Positive = Violet/blue

Ex. Staphylococcus aureus

Gram Negative = Red (Safranin dye)

Ex. Escherichia coli

Morphologies

Straight rod – Escherichia

Spore forming rod – Clostridium

Spiral forms – spirochaeta

Coccus – staphylococcus

Diplococcus – Neisseria

Chain – Streptococcus

Cluster – Staphylococcus

Bacillus = rod

Peptidoglycan Layer Differences

E. Coli – gram negative

Thick outer membrane

Staph aureus – gram positive

Thick peptidoglycan layer

Other ID Tests

KOH – dissolves host cells and bacteria; only fungal elements survive

Hyphae, spores

Acid Fast Stain – Mycobacterium spp., e.g Tuberculosis

Waxy mycolic layer surrounding cell wall; heat fix smear

India Ink Stain – “negative stain”; organism white, background black;

E.g. Cryptococcus

“Atypicals” - Lack characteristic structural components (cell wall) or metabolic capabilities. This separates them from the larger group of typical bacteria.

Mycoplasmas, Chlamydia, Rickettsias

Stain poorly, require special stains, or don’t stain

Metabolic Characteristics (oxygen use, growth req)

Aerobes: require O2 for energy and growth

Mainly at top of tube

Anaerobes: require no O2. Produce energy by fermentative pathways.

Mainly at bottom of tube

Facultative anaerobes: organism that preferentially use O2 for energy and growth, but may rely on anaerobic respiration depending on amount of O2 or fermentable material in environment

Problems arise when host defenses are compromised or bacteria migrate from the surface into sterile areas. In these cases, colonization may progress into invasion, infection and/or intoxication. Balance is key.

It is important to consider the status of your human host when collecting a history.

Is their immune status competent or incompetent?

Temporarily compromised?

What unusual exposure have they encountered?

Occupational – healthcare, TB

Recreational – travel, malaria

Pets – dog bites, cat bites

Social – IV drug use, skin popping -> cellulitis, endocarditis

Sexual partners – one or many?

Exchange sex for drugs or money?

Benefits?

Minimize survival of invaders

Secrete antimicrobial substances (GI)

Stimulus for development of immunity

Source of nutrients in the gut (vitamin k)

When does friend become foe?

Microbe is displaced from “home” and enters sterile site (IVDU – staph epidermidis)

Loss of balance – populations of harmless bacteria are reduced and lose their competitive advantage

Impaired immunity of host

Bacterial Pathogenesis

Entry to host/evasion of primary defenses

Adhesion of microbe to host cell

Propagation of the microbe

Damage to host by toxins; evoke inflammatory response

Evasion of host's secondary defenses

Evasion of immune system (surface antigen switching)

Bacterial Culture & Sensitivity

Why? Identifying microorganisms that cause disease and evaluating their susceptibilities allows us to tailor (narrow) therapy

What do we send? Body fluids or tissue

What do we get back? Identification of organisms – a preliminary report of morphology (24h), followed by a specific microbe (48-72h) along with a list of drugs that may or may not kill it (sensitivity or susceptibility report)

What’s the most important part of the process when evaluating and considering treatment of bacterial infection? The patient’s H&P!

The single most important source to consult when investigating an infectious disease problem is? The patient!

2 comments:

I would be awesome if I could get a version of this that is printable! Is that possible?? I just to the PACKRAT and see that my pathogen skills are lacking, this would be the perfect study material. Thank!